Recently, the spindle of a machine tool must be able to rotate at a very high speed, and thus conventionally, a motor for rotating at a medium speed is used and an increased revolutionary speed is obtained by using a transmission mechanism and thus the spindle is driven at a high revolutionary speed.
Nevertheless, problems arise such as a time lag in the revolution of the spindle when the revolution speed of the spindle is changed, and so on, whereby the finishing accuracy of a workpiece is reduced. Therefore, a spindle motor directly connected to the spindle of a machine tool is required, and thus a journal structure capable of enduring a high revolutionary speed becomes necessary. A journal bearing under a conventional grease lubrication becomes overheated at a high revolutionary speed such as represented by the value Dmn, which is obtained by multiplying an average diameter of an inner diameter and an outer diameter of the journal bearing by the rotational speed, and is substantially equal to or more than one million (for example, where the average diameter is 50 mm and the rotational speed is twenty thousand r.p.m.), and normal driving cannot be maintained. Therefore, an oil-air mix lubrication which supplies air containing an oil mist is needed.
Although the oil-air mix lubrication is needed for a normal driving of a motor at high revolutionary speeds, as described hereinafter, it is difficult to provide a channel for supplying an oil-air mix and a drain channel for a complex oil-air mix lubrication in a conventional journal bearing structure mounted in a onepiece type motor housing.